This invention relates to an antenna apparatus and in particular to a system for satellite tracking from an unstable platform.
This invention finds specific utilization in a ship borne satellite communication system having an antenna operated to track a satellite despite the motions of a ship.
Maritime communications are designed to provide ship-to-shore, and in some cases ship-to-ship communications, utilizing a communication satellite as a transmitting link. Given the environment of use, off-shore, the satellite antenna tracking system must be capable of prolonged, sustained operation, must be easily maintained and highly reliable.
Such systems first acquire through some form of external inputs as to position the desired communications satellite which is customarily placed in a stationary geosynchronous earth orbit. This requires, at a minimum, satellite elevation and azimuth data. Once the satellite has been acquired, the pointing attitude of the antenna is then continuously updated during the duration of the ship's voyage to maintain lock-on with the satellite irrespective of changes in the ships heading and position. Changes in the heading of the ship are generally automatically compensated in the azimuth axis, typically by direct link to the ship's compass. Position changes are relatively insignificant over short periods of time, for example, in stationkeeping operations. For a geosynchronous satellite, a one hundred mile change in the ship's position represents less than a 2.degree. tracking error, so changes occur gradually relative to changes in the ships position. To maintain lock when distances are traversed a tracking algorithm is used which constantly monitors signal strength and seeks the position at which it is maximized. Due to the nature of the algorithm it can only be used to correct long term errors and not rapid excursions.
A primary difficulty in maintaining lock-on with the satellite is the ship's motion, primarily pitch and roll disturbances. In a heavy sea state, rolling and pitching motion by wave action can be severe and sudden as well as the turning of the ship to a different, often inadvertent change of heading. Each of these motions require a change in the orientation of the antenna to maintain lock-on with the satellite. Additionally, motions of the ship are often quite sudden and therefore can be applied to the antenna with considerable force given its orientation displaced from the axis upon which motions of the ship occur.
It is customary to mount the antenna structure at the highest point on the ship to minimize reflections from the ship's superstructure and from the sea surface. These reflections tend to cause distortions or pertubations in the signals received from the satellite. While mounting at the highest point on the ship minimizes these disturbances and additionally tends to reduce interruption of received or transmitted signals which occur by having the signal path blocked by parts of the ship, the forces applied to the antenna are exacerbated at this location. That is, since the antenna is mounted at a position significantly removed from the origin of the axis of rolling, pitching and yawing of the ship, the actual translation of the antenna is amplified. Moreover, in the context of large vessels having engines, winches and the like, vibration is a factor. Consequently, the antenna structure must be configured to maintain a lock-on with the satellite irrespective of all of these external forces tending to move the antenna suddenly randomly and with great force.
Accepted techniques of maintaining antenna stability have been promised on first establishing a stabilized platform and then mounting the antenna on the stabilized platform. This technique generally uses gravity sensors, gyros, and accelerometers to determine on a real time basis motions of the ship. Gears and the like are then utilized to maintain a platform in a local horizontal plane irrespective of motions of the ship beneath it. The antenna mounted on the platform can then track the satellite irrespective of movement of the ship. This technique is shown with variations in terms of active sensors in U.S. Pat. Nos. 3,893,123, 3,999,184, 4,020,491, 4,035,805, 4,118,707. In these systems with a stabilized pedestal, the antenna is then configured for motion by elevation over azimuth. In such a system, shown in FIG. 1, as the ship rotates under the antenna, the azimuth axis compensates for ship's heading changes. Azimuth axis correction is therefore 360.degree.. Motion in the elevation axis is generally 90.degree.. It can be appreciated that the elevation over azimuth techniques is derived from ground based antenna systems where the mount can be leveled and fixed.
A recognized problem with this system is that as the ship rotates and azimuth corrections are made the connecting feed cables tend to wrap around the antenna mast. In order to continue operations, these cables must be periodically unwrapped by antenna rotation before it can continue tracking the satellite. Thus, a loss in communications results when this unwrapping process occurs.
A more serious problem is the complexity and weight associated with this system. Gravity sensors, accelerometers and gyroscopes used to provide sensor inputs for the stabilized platform are expensive and not considered to be highly reliable elements in the harsh off-shore environment. Moreover, each of the elements must be counterweighted such that the pedestal itself is balanced and then the antenna system on top of the pedestal is also balanced. Such systems tend to be relatively heavy, about 300-400 pounds for the stabilized platform and about 700-800 lbs for the complete system including a 4 ft. antenna including the radome. Given the fact that this entire apparatus is mounted on a pedestal above the ship superstructure, it is then also necessary to, in some cases, reballast the ship to avoid excessive rolling. Given the complexity and weight of such systems, there usage has generally been confined to large ships capable of carrying and supporting such systems.
In an attempt to eliminate complexity, but not necessarily weight, a second technique has been to define a passive stabilized platform upon which the antenna is mounted. Thus, rather than utilizing active sensors on the platform, the pedestal is mounted on a universal joint which is heavily ballasted to establish a pendulum type structure. In order to provide stability along one axis and thereby decouple motion in one direction, it is customary to mount a pair of counterrotating momentum wheels on the pedestal. A pair of momentum wheels is necessary to cancel out the torque which would be generated by the single unit. Such a passive system eliminates the complexity and aspects of unreliability in prior art active systems, however, the weight penalty remains. Moreover, passive stable platform utilizes the same elevation over azimuth motion having the wire wrapping problem as defined relatively to an active stabilized platform.